Alzheimer's disease is a common, progressive, incurable and ultimately fata disease of the central nervous system. At the histopathological level it is characterized by the formation of neurofibrillary tangles (NFT) and senile plaques (SP). At the ultrastructural level, unusual highly insoluble paired helical filaments (PHF) are detected. We have recently shown that brains from patients with Alzheimer's disease exhibit striking and unusual ubiquitin immunoreactivity on neurofibrils in regions containing NFT and SP. Very small numbers of ubiquitin immunoreactive neurofibrils are seen in normal aged brain, and such profiles are absent from younger healthy brain tissue. In preliminary experiments we have defined a proteolytic fragment from paired helical filament (PHF) preparations, believed to be the essential constituents of NFT, which has ubiquitin immunoreactivity, but which is much larger than ubiquitin. This must be derived from a protein in PHF fractions which is ubiquitinylated. We wish to follow up these interesting observations. We will: 1) use immunlogical, biochemical and molecular biological techniques to identify the parent molecule from which the ubiquitin-immunoreactive proteolytic fragment is derived. We will then know if this protein is present or absent in normal brain, and if it is present, if it corresponds to a normal cellular protein. 2) study the distribution of ubiquitin immunoreactivity, immunoreactivity for the ubiquitin acceptor protein, and immunoreactivity for other known or putative NFT markers to build up a comprehensive view of the relationship of each of these markers to one another in NFT and PHF. These studies will define in molecular terms an ubiquitinylated component of NFT and will set the stage for an examination of the mechanisms of PHF formation, the significance of ubiquitinylation in PHF formation, and the reasons why the ubiquitinylated NFTs are not degraded.